Projection aligner

ABSTRACT

A projection aligner includes a light source, a condenser lens system for directing light from the light source onto a mask on which a circuit pattern is formed, a projecting lens system for collecting on the surface of a wafer the light transmitted through the mask, and an aperture member disposed between the light source and the condenser lens system. The aperture member includes a transmitting zone for transmitting light from the light source and a phase shift member for producing a predetermined phase difference between light transmitted through the transmitting zone and light transmitted through a region surrounding the transmitting zone.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a projection aligner for use in theprocess of manufacturing LSIs.

2. Description of the Related Art

FIG. 20 shows a conventional projection aligner. A fly-eye lens 3 isdisposed diagonally to a lamp house 1. A mirror 2 is disposed betweenthe lens 3 and the house 1. An aperture member 4 is positioned in frontof the fly-eye lens 3. Condenser lenses 5 and 6, a mirror 7, and anexposure mask 8 on which a desired circuit pattern is formed arearranged along an optical path. A wafer 10 is situated in front of themask 8, and a projecting lens system 9 is disposed between the mask 8and the wafer 10.

As shown in FIGS. 21 and 22, the aperture member 4 has a disk-likeconfiguration with a circular opening 4a at the center thereof.

Light emanating from the lamp house 1 reaches the fly-eye lens 3 throughthe mirror 2, and is split into light beams which pass through lenses3a, of the fly-eye lens 3. The light transmitted through the respectivelenses 3a pass through the opening 4a of the aperture member 4, thecondenser lens 5, the mirror 7 and the condenser lens 6, and thenirradiate an exposure zone of the mask 8. The light beams transmittedthrough the lenses 3a of the fly-eye lens 3 are superposed on each otheron the surface of the mask 8, and thus the beams irradiate uniformly thesurface of the mask 8. In this way, the light beams pass through themask 8 and reach the wafer 10 through the projecting lens system 9,whereby the circuit pattern is imaged on the surface of the wafer 10.

It is known that the minimum resolution R of such a projection aligneris proportional to λ/NA, where λ is the wavelength being used and NA isthe numerical aperture of the optical system. Thus, the optical systemhas hitherto been designed so that the numerical aperture is increasedto improve the resolution of the projection aligner. In recent years theimproved resolution copes with a higher degree of integration of LSIs.

It is also known that as the numerical aperture of an optical systemincreases, the minimum resolution R decreases and the depth of focus(DOF) of the projection aligner also decreases even more than theresolution R does. The DOF is proportional to λ/NA². For this reason, inthe conventional projection aligner, the DOF decreases with an increasein the resolution, and the accuracy in transferring transcribing thecircuit pattern deteriorates.

SUMMARY OF THE INVENTION

The present invention has been made to solve such a problem.Accordingly, the object of the invention is to provide a projectionaligner increasing resolution and enlarging the depth of focus.

In order to achieve the above object, according to this invention, thereis provided a projection aligner comprising: a light source; a condenserlens system for directing light from the light source onto a mask onwhich a circuit pattern is formed; a projection lens system forcollecting on the surface of a wafer the light transmitted through themask; and an aperture member disposed between the light source and thecondenser lens system wherein the aperture member includes atransmitting zone for transmitting the light emanating from the lightsource and a phase shift member for producing a predetermined phaseshift between light transmitted through the center of the transmittingzone and light transmitted through a region surrounding the transmittingzone.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing the optical system of a projection aligner inaccordance with a first embodiment of the present invention;

FIG. 2 is a plan view showing an aperture member used in the firstembodiment;

FIG. 3 is a cross-sectional view of the aperture member shown in FIG. 2;

FIG. 4 is a view showing the distribution of the intensity of light onthe surface of a wafer when the light is completely focused thereon;

FIG. 5 is a view showing the distribution of the intensity of light onthe surface of the wafer when the light is not focused thereon;

FIG. 6 is an enlarged view showing the surface of the wafer when thelight is not focused thereon;

FIG. 7 is a view showing the result in which an optical image on thesurface of the wafer is simulated when the width of a phase shift memberof the aperture member is changed;

FIG. 8 is a cross-sectional view showing an aperture member used in asecond embodiment;

FIG. 9 is a plan view showing an aperture member used in a thirdembodiment;

FIG. 10 is a cross-sectional view of the aperture member shown in FIG.9;

FIG. 11 is a cross-sectional view showing an aperture member used in afourth embodiment;

FIG. 12 is a view showing the optical system of a projection aligner inaccordance with a fifth embodiment of the present invention;

FIG. 13 is a plan view showing an aperture member used in the fifthembodiment;

FIG. 14 is a cross-sectional view of the aperture member shown in FIG.13;

FIG. 15 is a view showing the distribution of the intensity of light onthe surface of the wafer when the light is not focused thereon accordingto the fifth embodiment;

FIG. 16 is a cross-sectional view of an aperture member used in a sixthembodiment;

FIG. 17 is a plan view showing an aperture member used in a seventhembodiment;

FIG. 18 is a cross-sectional view of the aperture member shown in FIG.17;

FIG. 19 is a cross-sectional view showing an aperture member used in aneighth embodiment;

FIG. 20 is a view showing the optical system of the conventionalprojection aligner;

FIG. 21 is a plan view of an aperture member used in the aligner shownin FIG. 20; and

FIG. 22 is a cross-sectional view of the aperture member shown in FIG.21.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments of the present invention will be described below withreference to the accompanying drawings.

FIG. 1 is a view showing the optical system of a projection aligner inaccordance with a first embodiment of the invention. A fly-eye lens 13is disposed diagonally to a lamp house 11 from a lamp which emits lighthaving a wavelength of λ, and a mirror 12 is disposed between the lens13 and the house 11. An aperture member 21 is positioned in front of thefly-eye lens 13. Condenser lenses 15 and 16, a mirror 17, and anexposure mask 18 on which a desired circuit pattern is formed arearranged along an optical path. A wafer 20 is situated in front of themask 18, and a projecting lens system 19 is disposed between the mask 18and the wafer 20.

As shown in FIGS. 2 and 3, the aperture member 21 has a disk-like outerframe 22 and an annular phase shift member 23 having a width W. Acircular opening 22a having a radius A is formed at the center of theframe 22. The phase shift member 23 is formed around the periphery ofthe opening 22a. The outer frame 22 is formed of a light-intercepting,i.e., opaque member such as metal. The opening 22a forms a transmittingzone D through which light from the lamp house 11 is transmitted. Thephase shift member 23 is formed of, for example, SiO₂, and is alsoformed to such a thickness that there is a phase difference of ahalf-wavelength, λ/2, between light transmitted through the center ofthe transmitting zone D, where there is no phase shift member 23present, and light transmitted through the phase shift member 23.

The operation of this embodiment will now be explained. Light from thelamp house 11 reaches the fly-eye lens 13 through the mirror 12, and issplit into light beams which pass through lenses 13a, which constitutethe fly-eye lens 13. The light beams transmitted through the respectivelenses 13a pass through the transmitting zone D of the aperture member21, the condenser lens 15, the mirror 17 and the condenser lens 16, andthen irradiate an exposure zone of the mask 18. The light beamstransmitted through the lenses 13a of the fly-eye lens 13 are superposedon each other on the surface of the mask 18, and thus the beamsirradiate uniformly the surface of the mask 18. In this way, the lightbeams pass through the mask 18 and reach the wafer 20 through theprojecting lens system 19, whereby an image of the circuit pattern isformed on the surface of the wafer 20.

As shown in FIG. 1, because of the phase shift member 23 formed at theperiphery of the transmitting zone D of the aperture member 21, thephases of light beams L2 and L3 transmitted through the phase shiftmember 23 of the aperture member 21 are reversed with respect to thephase of light beam L1 transmitted through the center of thetransmitting zone D. Therefore, when these light beams L1-L3 collect onthe surface of the wafer 20, the light beams L2 and L3, each in areverse phase, interfere with the light beam L1, thus offsetting eachother.

FIG. 4 is a view showing the distribution of the intensity of light onthe surface of the wafer when the light beams are completely focusedthereon. If the aperture member 21 were not provided with the phaseshift member 23, the distribution would be formed as indicated by abroken line 25. In this embodiment, however, because the aperture member21 is provided with the phase shift member 23, the light beam L1 isoffset by a reverse component 26 of the light beams L2 and L3, and thusthe distribution of the intensity of light is formed as indicated by asolid line 24. It is thus proved that when the lights are completelyfocused, because of the provision of the phase shift member 23, there isa decrease in the intensity of light, but there is substantially nodeterioration in the shape of the distribution of the intensity oflight.

On the other hand, when the light are beams not focused on the wafer 20as shown in FIG. 6, the light beams L1-L3 do not converge at one pointon the surface of the wafer 20. Thus, as indicated by dot chain lines ofFIG. 5, the reverse components of light beams L2 and L3, transmittedthrough the phase shift member 23, are distributed only around thecenter of a portion where the light beam L1 is most intense. For thisreason, only the intensity of light around a distribution line formedwhen the aperture member 21 is not provided with the phase shift member23 is offset by reverse components 29 and 30 of the light beams L2 andL3. In reality, the intensity of light is distributed as indicated by asolid line 27. For this reason, when the light beams are defocused, thatis, when the light beams L1-L3 do not converge at one point on thesurface of the wafer 20, only the intensity of light around a defocusedimage decreases. Then the image contrast is improved, so that itapproaches the level of image contrast when the light beams are focused.In other words, an image of sharp contrast is obtainable in a wideoptical axis direction, and the depth of focus (DOF) is enlarged.

The projection aligner shown in FIG. 1 permits enlargement of the DOF,as described above, while at the same time increasing the numericalaperture NA.

The ratio of the width W of the phase shift member 23 to the radius A ofthe transmitting zone D of the aperture member 21 is changed and anoptical image contrast on the surface of the wafer 20 is simulated. FIG.7 shows the results of the simulation. The abscissa of FIG. 7 indicatesthe degree of focus, the amount of defocus increasing toward the right.The ordinate indicates the width of a 25% distribution of the maximumintensity Ip, that is, one-fourth of the Ip of the optical image, asshown in FIG. 4. Symbol B₀ in FIG. 7 indicates an example of anallowable value of the width of the optical image when the circuitpattern is transferred. An optical image having a width B not greaterthan the allowable value B₀ is required for accurate transferral.

In the case of W/A=0, that is, when the phase shift member 23 is notprovided, as indicated by a broken line 31, the width B of the opticalimage increases sharply with an increase in the degree of defocusing,whereas the contrast decreases. In the case of W/A=5%, as indicated by asolid line 32, even when there is some increase in the degree ofdefocusing, the optical image exhibits a constant value which issubstantially equal to the width B when the light beams are completelyfocused. The value the increase, exceeding the allowable value B₀. Inthe case of W/A=10%, as indicated by a dot chain line 33, as the degreeof defocus increases, the width B of the optical image first decreases,and then increases and exceeds the allowable value B₀. The results ofsuch a simulation prove that the use of the aperture member 21satisfying a relationship W/A=5% is effective in accurately and stablytransferring the circuit pattern.

The phase difference between light transmitted through the center of thetransmitting zone D and light transmitted through a portion around thetransmitting zone D, due to the phase shift member 23, is not limited toa half-wavelength. However, as described in the embodiment mentionedabove, the use of the half-wavelength is the most effective in enlargingthe DOF.

FIG. 8 shows an aperture member 41 used in a second embodiment. Theaperture member 41 is constructed in such a way that an outer frame 42and a phase shift member 43 are formed on a crystalline substrate 44. Inthis embodiment, the deposition of SiO₂ on the crystal substrate 44permits easy formation of the phase shift member 43.

FIGS. 9 and 10 both show an aperture member 51 used in a thirdembodiment. The aperture member 51 is constructed in such a manner thata circular phase shift member 53 is formed at the center of a circularopening 52a in an outer frame 52. The phase shift member 53 is formed ona crystalline substrate 54. In the aperture member 51, as opposed to theaperture member 21 shown in FIGS. 2 and 3, the phase of lighttransmitted through the center of a transmitting zone D is reversed withrespect to the phase of light transmitted through a portion around thezone D. The same advantage as that described with the aperture member 21is obtainable.

FIG. 11 shows an aperture member 61 used in a fourth embodiment. Theaperture member 61 is constructed in the following way. First, an outerflame 62 and a phase shift member 63 are formed on a crystallinesubstrate 64. Then, antireflection films 65 and 66 made of, for example,MgF₂, are formed on the phase shift member 63 and the crystallinesubstrate 64 exposed outside. The formation of the antireflection films65 and 66 reduces the amount of stray light, thus resulting in improvedresolution and contrast of an image. The antireflection film may beformed on either the obverse or the reverse surface of a transmittingzone D. In FIG. 11, although the antireflection films are formed on theaperture member with the structure shown in FIG. 8, they may be providedon any of the aperture members with the structures described above. Thesame advantage as that described above is obtainable.

FIG. 12 is a view showing the optical system of a projection aligner inaccordance with a fifth embodiment of this invention. In thisembodiment, an aperture member 71 is used in place of the aperturemember 21 of the optical system in the first embodiment shown in FIG. 1.As illustrated in FIGS. 13 and 14, the aperture member 71 has a circularouter frame 72, an annular phase shift member 73, and an annularlight-intercepting member 74. A circular opening 72a having a radius Ais disposed at the center of the outer frame 72. The phase shift member73 having a width W₁ is disposed around the periphery of the opening72a. The light-intercepting member 74 having a width W₂ is formed aroundthe inside periphery of the phase shift member 73. In other words, thecenter of a transmitting zone D is separated by the light-interceptingmember 74 from a portion around the zone D. The outer frame 72 and thelight-intercepting member 74 are made of an opaque material, such asmetal, which blocks light. The phase shift member 73 is made of, forinstance, SiO₂.

When the aperture member 71 having a such a light-intercepting member 74is used, as indicated by dot chain lines of FIG. 15, when the lights aredefocused, the centers of reverse components 36 and 37 of light beams L2and L3, transmitted through the phase shift member 73, will deviate inan amount equal to a predetermined distance W₃ from components 38 of alight beam L1 which are positioned most outwardly. For this reason, onlylight in a portion around a distribution line formed when the aperturemember 71 is not provided with the phase shift member 73 is offset bythe reverse components 36 and 37 of the light beams L2 and L3. Inreality, the intensity of light is distributed as indicated by a solidline 34. Thus, when the light beams are defocused, only the intensity oflight around a defocused image decreases, and the image contrast isconsiderably improved, so that it approaches the level of image contrastwhen the light beams are completely focused. In other words, an image ofsharp contrast is obtainable in a wide optical axis direction, and thedepth of focus (DOF) is enlarged.

The ratio W₂ /A of the width W₂ of the light-intercepting member 74 tothe radius A of the transmitting zone D of the aperture member 71 waschanged and a circular contact hole pattern-exposed. It was found thatwhen the ratio W₂ /A is about 3%, the circuit pattern can be most stablytransferred.

As shown in FIG. 16, it may also be possible to employ an aperturemember 81 constructed in such a way that an outer frame 82, a phaseshift member 83, and a light-intercepting member 84 are formed on onecrystalline substrate 85. In such a case, the deposition of SiO₂ andmetal on the crystalline substrate 85 permits easy formation of thephase shift member 83 and the light-intercepting member 84.

Alternatively, as in an aperture member 91 shown in FIGS. 17 and 18, acircular phase shift member 93 may be formed at the center of a circularopening 92a in an outer frame 92 and a light-intercepting member 94 maybe formed around the phase shift member 93. The phase shift member 93and the light-intercepting member 94 are formed on a crystallinesubstrate 95.

As in an aperture member 101 depicted in FIG. 19, the formation ofantireflection films 106 and 107 made of, for example, MgF₂, reduces theamount of stray light, thus resulting in improved resolution andcontrast of an image. In the aperture member 101, first, an outer frame102, a phase shift member 103, and a light-intercepting member 104 areformed on a crystalline substrate 105. Then, the antireflection film 106is formed on the phase shift member 103, the light-intercepting member104 and the obverse side of the crystalline substrate 105, and theantireflection film 107 is formed on the reverse side of the crystallinesubstrate 105. The antireflection film may be formed on either theobverse or the reverse surface of a transmitting zone D. In FIG. 19,although the antireflection films are formed on the aperture member withthe structure shown in FIG. 16, they may be provided on an aperturemember with a different structure. The same advantage as that describedabove is obtainable.

What is claimed is:
 1. A projection aligner comprising:a light sourcefor producing light; a condenser lens system for directing light fromsaid light source onto a mask on which a circuit pattern is formed; aprojecting lens system for collecting on the surface of a wafer thelight transmitted through said mask; and an aperture member disposedbetween said light source and said condenser lens system wherein saidaperture member includes a transmitting zone for transmitting light fromsaid light source and a phase shift member for producing a predeterminedphase difference between light transmitted through the transmitting zoneand light transmitted through a region surrounding the transmittingzone.
 2. A projection aligner as claimed in claim 1 wherein said phaseshift member is an annular member defining a periphery of thetransmitting zone of said aperture member.
 3. A projection aligner asclaimed in claim 1 wherein said phase shift member produces a phasedifference of one half-wavelength.
 4. A projection aligner as claimed inclaim 1 wherein said phase shift member is SiO₂.
 5. A projection aligneras claimed in claim 1 wherein said aperture member includes an annularlight-intercepting member separating the transmitting zone from theregion surrounding the transmitting zone.
 6. A projection aligner asclaimed in claim 5 wherein the transmitting zone is circular and has aradius A, said light-intercepting member is annular, has width W₂, andis concentric with the transmitting zone, and the ratio W₂ /A issubstantially 3%.
 7. A projection aligner as claimed in claim 5 whereinsaid light-intercepting member is metal.
 8. A projection aligner asclaimed in claim 1 wherein said aperture member includes a transparentsubstrate, an outer frame disposed on said transparent substrate andhaving a central opening defining the transmitting zone, and said phaseshift member is disposed on the transparent substrate within the openingof said outer frame.
 9. A projection aligner as claimed in claim 8wherein said transparent substrate is quartz, and said outer frame ismetal.
 10. A projection aligner as claimed in claim 1 wherein saidaperture member includes an antireflection film within the transmittingzone.
 11. A projection aligner as claimed in claim 10 wherein saidantireflection film is MgF₂.
 12. A projection aligner as claimed inclaim 1 wherein the transmitting zone is circular and has a radius A,said phase shift member is annular and has width W₁, and the ratio W₁ /Ais substantially 5%.
 13. A projection aligner comprising:a light sourcefor producing light; a condenser lens system for directing light fromsaid light source onto a mask on which a circuit pattern is formed; aprojecting lens system for collecting on the surface of a wafer thelight transmitted through said mask; and an aperture member disposedbetween said light source and said condenser lens system wherein saidaperture member includes a transmitting zone for transmitting light fromsaid light source and a phase shift member within the transmitting zonefor producing a predetermined phase difference between light transmittedonly through the transmitting zone and light transmitted through thetransmitting zone and the phase shift member.
 14. A projection aligneras claimed in claim 13 wherein said phase shift member is a disc-likemember centrally disposed within the transmitting zone of said aperturemember with an annular region of the transmitting zone surrounding saidphase shift member.